Selective Hydrosilation of CO sub(2) to a Bis(silylacetal) Using an Anilido Bipyridyl-Ligated Organoscandium Catalyst

A rigid anilido bipyridyl ligand has been designed for use in an organoscandium-based carbon dioxide hydrosilation catalyst. Ligand attachment by alkane elimination results in metalation of an aryl C--H bond in a 3,5-di-tert-butylphenyl group installed on the pyridyl unit, thus rendering the ligand tetradentate. Insertion of CO sub(2) into the newly formed Sc--C bond leads to a [kappa] super(1) carboxylate which, when treated with the borane B(C sub(6)F sub(5)) sub(3), becomes hemilabile. In addition to activating the catalyst, the k super(1) carboxylate effectively sequesters free B(C sub(6)F sub(5)) sub(3) and the ensemble is able to effectively hydrosilate CO sub(2), in the presence of excess Et sub(3)SiH, almost exclusively to R sub(3)SiOCH sub(2)OSiR sub(3). A maximum turnover number of about 3400 (conversion of silane) is observed. Mechanistic experiments suggest that the sequestration of free B(C sub(6)F sub(5)) sub(3) by the hemilabile carboxylate contributes to the selectivity observed and prevents over reduction to methane. Lewis acid shuffle: Selective hydrosilation of CO sub(2) to a bis(silylacetal) is mediated by a scandium-based catalyst activated by B(C sub(6)F sub(5)) sub(3). The selectivity derives from the ability of the scandium complex to moderate the propensity of borane to further hydrosilate the Et sub(3)SiOCH sub(2)OSiEt sub(3) product by limiting the amount of free B(C sub(6)F sub(5)) sub(3) present in solution.